The present invention is directed to a device and a process for the production of electrophotographic images. More specifically, the present invention is directed to a shutter-type image bar that utilizes an electrochromic switching system and is suitable for use in an electronic copier or printer.
Shutter-type optical image bars are arrays of pixel sized shutters controlling the light output of a uniform source. A typical shutter-type image bar constitutes a single or double line array of electrically addressed shutters that either do or do not allow light to pass. The arrays are used to expose a photosensitive medium, such as a charged photoreceptor. Thus, a latent electrostatic image is formed on the photosensitive medium by passing light from a uniform light source through the image bar and onto the photosensitive medium, thereby selectively exposing and discharging certain areas on the medium.
Many known shutter-type image bars contain liquid crystal systems, wherein the shutters comprise individually addressed liquid crystal devices that either allow light to pass through them or exist in an opaque state that does not permit the passage of light, depending on the voltage applied to each shutter. Liquid crystal devices are generally not color selective, and are suitable only for black and white light shuttering, and thus black and white printing. However, certain special complex cell configurations using a plurality of different liquid crystalline materials do allow for multicolor applications, as disclosed in T. Uchida, Mol. Cryst. Liq. Cryst. 123 (1-4), 15-55 (1985). Liquid crystal image bars tend to be disadvantageous in that many layers of different liquid crystalline materials are required for color selective shuttering to enable colored image exposure of a color selective photoreceptor. Liquid crystal optical image bars also generally possess disadvantages such as the requirement of AC voltage for operation, because of the limited useful life of liquid crystal devices used in conjunction with DC power sources, with the accompanying necessity for complicated AC drive circuitry instead of relatively simple DC drive circuitry. Liquid crystal devices also require the application of power to maintain an image, since the alignment and coloration of the crystalline material in the shutters of the imaging bar will be lost as soon as voltage is removed. In addition, liquid crystal devices generally exhibit relatively slow switching times of about one millisecond, although microsecond switching times are possible in special systems, such as that disclosed in N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., vol. 36, pages 899-901 (1980).
Electronic imaging devices with imaging bars are known. For example, U.S. Pat. No. 4,374,397, the disclosure of which is totally incorporated herein by reference, discloses an electronic imaging apparatus using light valve devices having improved addressing configurations and modes. The apparatus has a locationally addressing light valve array having a plurality of location-address sectors, each sector including a plurality of discrete light valve pixel portions which are electronically addressable to control the passage of light. Each discrete pixel portion is electrically coupled to one corresponding light valve pixel portion of each of the other sectors, but is independent of other light valve pixel portions in its own sector.
Further, U.S. Pat. No. 4,595,259, the disclosure of which is totally incorporated herein by reference, discloses a liquid crystall image bar for an electrophotographic printer. The image bar has a nematic liquid crystal device with crossed polarizers configured as a single array of dot shutters that are driven by a single frequency voltage source. The image bar produces latent electrostatic images on the printer's photoconductive member one line at a time in response to receipt of digitized data signals. During the operational mode, all of the image bar shutters are energized and, in this electrically driven state, the image bar shutters are non-transmissive. The latent image is produced by selective erasure of precharged background areas on the member. For the erasure to occur, interruption of the driving voltage to selected shutters in response to the data signals causes the selected shutter to become transmissive because the liquid crystal material enters a transient state. The response time for the transmissiveness is about one millisecond. Consecutively erased spots by the same shutter requires the momentary reapplication of the driving voltage.
Another patent, U.S. Pat. No. 4,375,647, the disclosure of which is totally incorporated herein by reference, discloses an apparatus for scanning successive lines of a color original moved past a scan station. The apparatus includes a photodetector means, an illuminating means for producing a plurality of discrete color separation illuminations of the detector via each passing original line, a light valve array having discretely activatable pixel portions aligned with the pixels of the scan station to control color separation illumination of the photodetector, and a scan address means for activating pixel portions of the array in a predetermined serial sequence and in timed relation with the movement of the original.
Additionally, U.S. Pat. No. 4,375,648, the disclosure of which is totally incorporated herein by reference, discloses a high resolution light valve apparatus for electronic imaging. The apparatus utilizes an area light valve array to effect high pixel resolution exposure of a line of record medium passing through an area exposure region during a plurality of partial line exposure subperiods. This apparatus provides improvements in electro-optic modulator and addressing electrode structure of light valve devices which provide for simplified, high resolution optical address of pixels. Also, U.S. Pat. No. 4,378,568, the disclosure of which is totally incorporated herein by reference, discloses a light valve imaging apparatus having a line of discretely addressable pixels that can be activated between a light transmitting and a light blocking state by an electrical field. The apparatus possesses improved structure and method for electrical addressing of the light valve apparatus.
Further, U.S. Pat. No. 4,449,153, the disclosure of which is totally incorporated herein by reference, is directed to an apparatus and method for light valve imaging wherein light is uniformly directed onto a linear light valve array, which includes pixel portions selectively activated to block or transmit light during successive line address periods. The patent discloses an improved gray scale imaging method wherein the intensity of the light passing to the light valve array is modulated during each line address period. In addition, U.S. Pat. No. 4,458,989, the disclosure of which is totally incorporated herein by reference, discloses an electro-optic addressing apparatus having an improved electrode configuration and an improved address control. During a first address stage, activating and inactivating reference potentials are applied respectively to first and second adjacent pixel portions of the modulator while an addressing signal potential is applied to both pixel portions. During a second address stage, activating and inactivating reference potentials are applied respectively to the second and first pixel portions while signal potential is applied to both pixel portions.
Another patent, U.S. Pat. No. 4,560,994, the disclosure of which is totally incorporated herein by reference, discloses a two dimensional electro-optic modulator for printing which comprises an electro-optic element and a two dimensional planar array of individually addressable electrodes for spatially modulating a light beam along a plurality of transverse axes. The modulator is useful for applications such as electro-optic line printing and optical displays.
In addition, U.S. Pat. No. 3,439,174, the disclosure of which is totally incorporated herein by reference, discloses an image forming mechanism in which a light image is transformed or converted to a semipermanent type of image. The image may be retained as long as is desired, and the mechanism can be reused. This mechanism is based on an electrolytic process and comprises a laminar sandwich arrangement of several layers among which are included a transparent substrate, a transparent electrode, a transparent photoconductor, an electrolyte layer, and a metallic anode. The light image is transformed by the deposition of a corresponding metallic image on the photoconductive layer. Also, U.S. Pat. No. 4,250,876, the disclosure of which is totally incorporated herein by reference, discloses a light amplification device employing transparent sheets between which an electric field is impressed and having a photoconductive sheet and an electrically optically active sheet such as a liquid crystal sheet. A lattice member is inserted between the photoconductive sheet and the liquid crystal sheet, and a gravure display results. Color displays are also possible with the disclosed apparatus.
Electrochromic materials such as those employed with the present invention are known. In general, electrochromic materials are those materials having at least two oxidation states and for which the material is of a different color in different oxidation states. The material shifts from one oxidation state to another when an electron is either removed (oxidation) or added (reduction) by an electric field applied to it. An electrochromic display, for example, operates by means of an electrochemically induced color change on an active film coating. A thin film is deposited on a conductive substrate and is used as an electrode in an electrochemical cell, where it can be reduced or oxidized. The film will have a reversible characteristic color change depending on the magnitude and polarity of the applied voltage.
Electrochromic and electro-optical devices are also known. For example, U.S. Pat. No. 3,521,941 discloses an electro-optical device comprising a layer of a transition metal compound exhibiting persistent electrochromism and an insulating layer sandwiched between two conductive transparent electrodes. Coloration and bleaching occur in the device when the polarity of the applied electric field is controlled. The device disclosed in the patent provides an improvement over known devices by addition of the insulating layer to the electrode-electrochromic layer sandwich. This insulating layer allows rapid change of the electrochromic material with application of the electric field and rapid return to the original state. The electrochromic layer may be of an electrically insulating or semiconducting material having the ability to remain in the absorptive state to which it is changed after removal of the electric field, instead of instantaneously reverting to its initial state.
A similar electrochromic device is disclosed in U.S. Pat. No. 3,986,771, which teaches a device allowing storage and reproduction of an original light image consisting of an electrochromic layer and a photoconductive layer sandwiched between conductive transparent electrodes. The device allows for repeated storage and erasure of information on the information storage medium, which comprises an electrochromic material. In addition, the device possesses a means for projecting an original image onto a storage medium, and is capable of projecting an image stored on the storage medium onto a photosensitive material, so that copies may be made. Free modification or compiling of the original image by erasure or addition is also possible.
An electrochromic display device with the ability to maintain constant color density is disclosed in U.S. Pat. No. 4,479,121. The device comprises a plurality of display segments on one substrate in contact with an electrolyte, which in turn is in contact with the counterelectrode on the surface of a second sandwiching substrate. Each display segment comprises a transparent electrode covered by an electrochromic layer that exhibits either a colored or a bleached state, depending on the amount of electric charge held by the display segment. Color density is maintained constant by the injection or extraction of electric charge into or from the display segments, which compensates for the difference between the coloration electric charge quantity at the present display state and the charge quantity at the next display state.
Another patent, U.S. Pat. No. 4,110,014, discloses an apparatus for displaying information stored in the form of microimages and for presenting and recording time-sequential information such as television signals. A switching apparatus is coupled to a plurality of transparent conductive strips separated from each other such that each may assume its own electrical potential. The device includes an electro-optic sheet and a photoconductive sheet sandwiched between two planar X and Y sets of transparent conductive addressor strips. The X addressor strips are positioned at right angles to the Y addressor strips. In the parallel mode of operation, the addressor strips of each set carry the same potential, and a uniform potential is maintained between the X and Y electrodes. In the series mode of operation, the electro-optic sheet undergoes a change in optical property when locally impressed by an electric field. The X and Y addressor circuits are coupled to conventional series information sources, such as a television receiving system. Serial information may be recorded by uniformly exposing the photoconductor of the device and using the addressor strips to produce an image pattern in the electro-optic layer.
An electro-optical color imaging apparatus in which a plurality of discrete picture elements, or pixels, of a color sensitive imaging layer or layers are concurrently imagewise exposed is disclosed in U.S. Pat. No. 4,229,095. The apparatus is suitable for use with a multi-color mixture of photoconductive particles or other color sensitive imaging layers, and comprises a plurality of separately addressable electro-optical illuminating means which concurrently subject a plurality of picture elements of the imaging layer respectively to different color light pulses in accordance with the color content of the image to be formed.
Another publication, U.S. Pat. No. 3,589,896, discloses an electro-optical article employing electrochromic and photoconductive materials. The device comprises an electrochromic material sandwiched between electrodes, with an additional layer of a photoconductive material, such that the device can be used for computer generated display applications. This additional layer comprises a sustained bombardment induced conductivity (SBIC) material, which is similar to a photoconductor, but is also sensitive to electron beam excitation.
Additionally, U.S. Pat. No. 3,708,220 discloses an electro-optical device useful for controlling the absorption of visible and infrared radiation by windows, data display devices, and other items. The device comprises a pair of electrodes between which are situated two identical layers of transition metal electrochromic compounds separated by a semisolid highly conductive sulfuric acid gel electrolyte. Further, U.S. Pat. No. 3,840,288 discloses an electrochromic device having a layered structure comprising, in the order stated, a transparent conductive electrode, an electrochromic layer, an ion permeable insulator layer, an electrocatalytic layer, a sponge material, and an optional top electrode. The device allows for an all solid state display, and can be altered from the first absorption state to the second by the ionization of atoms or molecules in the electrocatalytic layer, which results in a reversible color change.
In U.S. Pat. No. 4,297,695, an electrochromic display device operated by selective application of electrical signals to first and second electrode matrix addressing means having a potential of signals sufficient to produce local color changes in an electrochromic material is disclosed. A color change is effected by a short "expose" pulse, provided by the first matrix, said pulse having a potential above the threshold required for coloration but with insufficient charge to cause coloration, followed by a second, longer "develop" pulse, provided by the second matrix, said pulse being below the coloration threshold potential.
Further, U.S. Pat. No. 4,426,643 discloses an electrolytic apparatus employing a reference electrode, such as an electrochromic display of the type employing potentiostatic control of write or erase operations. The apparatus has at least one electrode on which a predetermined amount of electrochromic or other electrodepositable material is maintained or replenished by intermittent connection of the electrode to a source of write current. According to this patent, a coated reference electrode of this type has a stable potential with respect to the solution, which is necessary for potentiostatic control.
Although the apparatuses and methods of the prior art are suitable for their intended purposes, a need exists for shutter-type image bars which can either fully discharge selected areas of a photoreceptor for the formation of single colored images or can modulate the level of photoreceptor discharge in a single pass, with the resulting voltage levels being selectively developed with different colored toners, for the formation of images of more than one color. There is also a need for an image bar with multi-color switching capability which can write a multi-color or full-color image on a photoreceptor, which photoreceptor has color selectivity in that it discharges to different levels of potential depending on the color of the exposing light. In addition, there is a need for an image bar with multi-color switching capability that contains a single active material. Further, a need exists for image bars capable of operating with DC voltage, such that they may possess a simple drive circuitry. In addition, a need exists for image bars with rapid switching times to enable high speed formation of images. A need also exists for image bars with low cost, low switching energy, and low voltage requirements that also provide images of high contrast. Further, a need exists for imaging bars wherein the individually addressable pixels retain their coloration without the need for a continuously applied electric field after the pixel has been switched to its desired state, in order to provide devices with lower energy consumption requirements as compared to devices containing liquid crystal image bars.